1. CAPRISA Treatment Research Program, Centre for the AIDS Programme of Research in South Africa (CAPRISA), University of KwaZulu-Natal, 4013 Durban, South Africa
2. Department of Public Health and Family Medicine, University of KwaZulu-Natal, 4013 Durban, South Africa
3. Department of Epidemiology, Mailman School of Public Health at Columbia University, NY 10032, USA